This is a continuation of our studies of the pathophysiology of the retinal, choroidal and optic nerve circulations. We propose to elucidate the processes involved in autoregulation of blood flow and oxygen transport in the eyes of both normal animals and in animals with experimentally induced pathology. The retinal circulation will be studied by microelectrodes which can be used to measure both tissue available oxygen and microregional blood flow by hydrogen washout from the retina. We will study retinal vascular reactivity (autoregulation) in normal cats and dogs. We will also study dogs made diabetic with alloxan, and kittens with experimentally induced retrolental fibroplasia. Vascular reactivity will be studied by measuring changes in oxygen tension and blood flow secondary to induced ocular hypertension, hyperglycemia, and 100 percent oxygen breathing. The choroidal circulation will be studied by blood flow measurements using microsphere impaction and indocyanine green dye washout. Outer retinal oxygen tension will be measured by fluorimetry using an intrinsic indicator (NAD-NADH) and an extrinsic indicator (PyreneButyric Acid). Normal cats will be used to study the effects of hyperglycemia and systemic hypertension on the choroidal circulation. The circulation in the distal segment of the optic nerve behind the lamina cribrosa will be studied using microelectrodes for both oxygen tension and microregional blood flow (by hydrogen washout). Intravascular pressures will be measured with micropipettes. Monkeys (cynomolgus) will be studied following induced ocular hypertension, hypotomy, and 100 percent oxygen breathing.